Usually it is when the pup (Kasey) starts nosing around when the older lab (Maddi) has found something to chew on.

That could be your common denominator. Some dogs are not comfortable with sharing and resource guard. I would start by separating your dogs when they have chew items. Its only fair to them to be able to chew in peace without the fear of another dog stealing there goodies

Thanks Snowshoe. We brought the puppy into the home when she was 8 weeks. Kasey (pup) has always treated Maddi (9 year old) like her best friend. They run, play, swim, share toys, tug chase, etc. They do play rough, growling pawing, but there is a definite difference when they get into it and not misguided play.

Thanks to all who posted to this thread. For some clarification, here is my wife's description.

Ok, let me try to clarify. When Kasey was first brought home (at 8 wks), for the first few months there was some correcting from the older labs. Mainly from the younger of the two older labs (Mocha - 7 yrs). Though it scared us at first, we allowed it and did not stop the behavior from the older dogs. Then about 3 months ago, some very aggressive behavior started. It has been between Kasey (18 months) and either Maddi ( 9 yrs) or Mocha. Kasey is always involved initially and usually the 3rd one comes in to back up her sister. Now to describe the aggressive behavior. Hackles are up and teeth are bared. The last time this happened, Maddi was bleeding from two small bite marks on her snout. I swear sometimes it looks like it started over nothing. Like two of them meet between a chair and maybe one didn't move out of the way?? Food, toys, chew toys, etc. do not have to be involved. We never have trouble at meal time and feed them oldest to youngest. Also, this isn't a daily event. Normally, they play rough, share toys and sleep together on our bed. P.S. Please ignore Bill's "bow up"!! I have been married to him for 30 yrs. and I have no clue what he is talking about!!!! :-)

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I do know that the younger will sometimes challenge the pack hierarchy. From what I've read it's very difficult to prevent this and it's better to go along with it.

Girls will sometimes fight, really fight, and I've known folks who have rehomed a beloved dog in order to prevent serious injury to one. There's a saying, "Dogs fight to breed, bitches fight to breathe."

You can find tons of stuff on dog forums about both the above two comments.

A little bit of blood in rough play isn't so unusual. Ripped flesh, lots of blood, a whirlwind fight so fast you can't even see which dog is where, happening too often is cause for concern. Without seeing and with me only ever having one dog at a time, can't say more. I wonder if you should get a behaviourist in to observe?